The utilization and digestion of cellulose by the rumen ciliate Diploplastron affine

Degradation of Cellulose and Hemicellulose by Ruminal Microorganisms

As major structural components of plant cell walls, cellulose and hemicellulose are degraded and fermented by anaerobic microbes in the rumen to produce volatile fatty acids, the main nutrient source for the host. Cellulose degradation is carried out primarily by specialist bacteria, with additional contributions from protists and fungi, via a variety of mechanisms. Hemicelluloses are hydrolyzed by cellulolytic bacteria and by generalist, non-cellulolytic microbes, largely via extracellular enzymes. Cellulose hydrolysis follows first-order kinetics and its rate is limited by available substrate surface area. Nevertheless, its rate is at least an order of magnitude more rapid than in anaerobic digesters, due to near-obligatory adherence of microbial cells to the cellulose surface, and a lack of downstream inhibitory effects; in the host animal, fiber degradation rate is also enhanced by the unique process of rumination. Cellulolytic and hemicellulolytic microbes exhibit intense competition and amensalism, but they also display mutualistic interactions with microbes at other trophic levels. Collectively, the fiber-degrading community of the rumen displays functional redundancy, partial niche overlap, and convergence of catabolic pathways that all contribute to stability of the ruminal fermentation. The superior hydrolytic and fermentative capabilities of ruminal fiber degraders make them promising candidates for several fermentation technologies.

Effects of Thymol Supplementation on Goat Rumen Fermentation and Rumen Microbiota In Vitro

This study was performed to explore the predominant responses of rumen microbiota with thymol supplementation as well as effective dose of thymol on rumen fermentation. Thymol at different concentrations, i.e., 0, 100 mg/L, 200 mg/L, and 400 mg/L (four groups × five replications) was applied for 24 h of fermentation in a rumen fluid incubation system. Illumina MiSeq sequencing was applied to investigate the ruminal microbes in addition to the examination of rumen fermentation. Thymol doses reached 200 mg/L and significantly decreased (p < 0.05) total gas production (TGP) and methane production; the production of total volatile fatty acids (VFA), propionate, and ammonia nitrogen, and the digestibility of dry matter and organic matter were apparently decreased (p < 0.05) when the thymol dose reached 400 mg/L. A thymol dose of 200 mg/L significantly affected (p < 0.05) the relative abundance of 14 genera of bacteria, three species of archaea, and two genera of protozoa. Network analysis showed that bacteria, archaea, and protozoa significantly correlated with methane production and VFA production. This study indicates an optimal dose of thymol at 200 mg/L to facilitate rumen fermentation, the critical roles of bacteria in rumen fermentation, and their interactions with the archaea and protozoa.

Single-cell transcriptome sequencing of rumen ciliates provides insight into their molecular adaptations to the anaerobic and carbohydrate-rich rumen microenvironment

Rumen ciliates are a specialized group of ciliates exclusively found in the anaerobic, carbohydrate-rich rumen microenvironment. However, the molecular and mechanistic basis of the physiological and behavioral adaptation of ciliates to the rumen microenvironment is undefined. We used single-cell transcriptome sequencing to explore the adaptive evolution of three rumen ciliates: two entodiniomorphids, Entodinium furca and Diplodinium dentatum; and one vestibuliferid, Isotricha intestinalis. We found that all three species are members of monophyletic orders within the class Litostomatea, with E. furca and D. dentatum in Entodiniomorphida and I. intestinalis in Vestibuliferida. The two entodiniomorphids might use H₂-producing mitochondria and the vestibuliferid might use anaerobic mitochondria to survive under strictly anaerobic conditions. Moreover, carbohydrate-active enzyme (CAZyme) genes were identified in all three species, including cellulases, hemicellulases, and pectinases. The evidence that all three species have acquired prokaryote-derived genes by horizontal gene transfer (HGT) to digest plant biomass includes a significant enrichment of gene ontology categories such as cell wall macromolecule catabolic process and carbohydrate catabolic process and the identification of genes in common between CAZyme and HGT groups. These findings suggest that HGT might be an important mechanism in the adaptive evolution of ciliates to the rumen microenvironment.